1. Field of the Invention
The invention relates to enclosures for electronic apparatus and, more particularly, to such enclosures that reduce electromagnetic interference emissions to and/or from the enclosures. The invention also relates to electronic apparatus employing enclosures that reduce electromagnetic interference emissions thereto and/or therefrom.
2. Background Information
The operation of electronic apparatus (e.g., without limitation, electronic equipment; electronic devices; televisions; radios; computers; medical and other electronic instruments; business machines; communications devices; control and/or monitoring devices) is attended by the generation of electromagnetic radiation within the electronic circuitry of the apparatus. Such radiation often develops as a field or as transients within the radio frequency band of the electromagnetic spectrum (e.g., between about 10 KHz and 10 GHz), and is termed “electromagnetic interference” (EMI), which is known to interfere with the operation of other proximate electronic devices. See, for example, U.S. Pat. Nos. 5,202,536; 5,142,101; 5,105,056; and 4,857,668.
Digital and/or processor-based electronic devices produce electromagnetic fields at harmonics of the clock frequency and also at frequencies related to the rise and fall times of logic signals. The most straightforward controls for EMI involve the use of multi-layer printed circuit boards and ground planes. Nevertheless, emissions can easily exceed regulatory requirements.
The last line of defense is the electronic enclosure. To reduce electromagnetic emissions, the typical electronic enclosure is made of conductive materials, such as aluminum or steel. Even then, any openings or seams may act as slot antennas. These slots pass EMI at frequencies having wavelengths less than eight times the size of the slot. To attenuate EMI effects, shielding having the capability of absorbing and/or reflecting EMI energy may be employed both to confine the EMI energy within a source device, and to insulate that device or other “target” devices from other source devices. Such shielding is provided as a barrier, which is inserted between the source and the other devices, and typically is configured as an electrically conductive and grounded housing to enclose the device.
As the circuitry of the electronic device generally must remain accessible for servicing or the like, most housings are provided with openable or removable accesses, such as doors, hatches, panels or covers. Between even the flattest of these accesses and its corresponding mating or faying surface, however, there may be present gaps, which reduce the efficiency of the shielding by presenting openings through which radiant energy may leak or otherwise pass into or out of the device. Moreover, such gaps represent discontinuities in the surface and ground conductivity of the housing or other shielding, and may even generate a secondary source of EMI radiation by functioning as a form of slot antenna In this regard, bulk or surface currents induced within the housing develop voltage gradients across any interface gaps in the shielding, which gaps thereby function as antennas and radiate EMI noise. In general, the amplitude of the noise is proportional to the gap length, with the width of the gap having a relatively smaller effect.
To address EMI, EMI shielding products attempt to prevent undesired electromagnetic energy and radio frequency interference (RFI) from disrupting, or radiating from, electronic devices. Such products include, for example, wire mesh O-rings; fabric-over-foam profile gaskets; shielding tapes; fabric-over-foam I/O gaskets; cable shielding; EMI shielding glass; shielding laminates; selectively coated, custom formable shields; beryllium copper spring-finger gasketing; and silver coated nylon gasketing. Nevertheless, such products are expensive.
One example of an electronic device is the personal computer (PC). In order to attenuate electromagnetic signals, the typical PC case includes steel plates and shields; nevertheless, there are emissions through the seams and through openings created for add-on cards. In some applications, electromagnetic gasket material attempts to attenuate non-complaint signals at the seams and the openings. The more overlap provided by the gasket material at a seam or opening, the better the attenuation. However, the cost of the gasket material and related assembly is relatively expensive.
In a microwave oven, for example, the wavelengths of signals are relatively very short. Hence, such microwave ovens employ electromagnetic gaskets for relatively long seams and an array of relatively small holes in a conductive panel for the window.
For filling gaps within mating surfaces of housings and other EMI shielding structures, gaskets and other seals have been proposed for maintaining electrical continuity across the structure. Such seals are bonded or mechanically attached to, or press-fit into, one of the mating surfaces, and close any interface gaps, in order to establish a continuous electrically conductive path thereacross by conforming under an applied pressure to irregularities between the surfaces.
EMI shielding gaskets, for example, are used in electronic equipment to provide protection against interference from electromagnetic energy, including RFI and more broadly all bands of EMI. The shielding has an electrically conductive element, be it a wire mesh, conductive filler or conductive plating, coating or fabric, which prevents external EMI from interfering with an electronic device and/or protects other adjacent electronic devices from EMI emitted by an electronic device.
The Background of the Invention section of U.S. Pat. No. 6,521,828 discloses a form-in-place FIP) process for the manufacture of EMI shielding gaskets. One method of achieving a lower closure force gasket design has been to form the gasket as having a periodic “interrupted” pattern of alternating local maxima and minima heights. Gaskets of such type may be formed by molding or the FIP process as having a crenellated, i.e., notched, serrated or a sinusoidal “waveform” profile, or as a series of discrete beads. In general, for a specified joint configuration, a gasket having such an “interrupted” profile or pattern would be expected to exhibit a greater deflection under a given compressive load than a continuous profile.
U.S. Pat. No. 5,259,792 discloses an electrical connector housing for a flat ribbon-type electrical transmission cable and method for minimizing EMI emissions. The connector housing has a top half and a bottom half, which are fitted together with an interlock joint, in order to minimize the emission of EMI through the joint. The interlock is provided by a number of interrupt elements that provide discontinuity along the otherwise continuous line joint to minimize the emissions of interference signals. Preferably, the interlock along the interface between the top and bottom connector housing halves is provided by serrations in the form of triangular teeth that fit together. The teeth interlock, in order that there is insufficient space to allow for the transmission of EMI at frequencies at least up to six gigahertz. However, the interrupt may be provided in the form of other elements as long as there is no space having a linear dimension greater than ⅛ of the EMI wavelength. The plastic housing halves are plated with a metal plating shield, such as a copper-nickel alloy, to provide shielding. Also, RFI shielding strips are affixed to each housing half, in order that when the housing halves are clamped together the shielding strips will make positive electrical contact with a cable foil metal shield.
Military contractors have used many techniques to attenuate and deflect radar signals. For example, the goal of stealth technology is to make an aircraft invisible to radar. The stealth aircraft is shaped, in order that any incident radar signals are reflected away from the radar source and/or the aircraft is covered in materials that absorb radar signals. For example, the stealth aircraft may be made up of completely flat surfaces and relatively very sharp edges. When a radar signal hits a stealth aircraft, the signal reflects away at an angle. In contrast, most conventional aircraft have a rounded shape, which creates a very efficient radar reflector, thereby reflecting some of the signal back to the source.
There is room for improvement in electronic apparatus and enclosures therefor.